The present invention relates to an inductive measuring transducer for determining the position of a body which is movable relative to another body, one of such bodies being equipped with at least one inductive sensor component that produces a local magnetic field and the other one of such bodies comprising an inductive pickup into which the local magnetic field is injected, and to an electronic circuit for supplying the inductive sensor component with an alternating voltage and a circuit for evaluating the output voltages of the sensor. It is the object of the invention to be described hereafter, to provide a measuring transducer which achieves a sufficiently high degree of precision at low expense.
Measuring transducers of that kind are used as measuring transducers preferably for measuring angles and displacements.
For converting an angular position of a rotor relative to a stator to an electric signal a large number of measuring methods have been known that can be implemented in the most different ways, depending on the particular application. With respect to insensitivity to environmental influences, the inductive measuring principle has been found to be especially well suited. Contrary to optical or capacitive methods, it is particularly insensitive to contamination and moisture condensation. Magnetic sensors may be influenced by external fields, or by aging or demagnetization of the permanent magnets employed. In addition, they are not very much suited for hollow-shaft sensors because normally the sensor has to be arranged at the center of rotation. Potentiometric angle sensors are connected with the disadvantage that the wipers and resistance layer are subject to wear.
Inductive angle sensors, such as resolvers or synchros, are known and have proven their value in practice. Because of their symmetric structure, the latter are relatively insensitive to eccentricity. On the other hand, however, only small gaps are possible between rotor and stator. Due to their structure, they are relatively expensive because precision-made and precision-wound stators and rotors lead to high production costs and material input.
DE 41 13 745 A1 describes a structure where a centrally arranged excitation coil magnetizes a pot-type ferrite core, fitted on the rotor shaft and comprising outer pot halves of different diameters, a circuit board with an induction loop being arranged between the outer pot halves, from which a voltage can be picked off by suitable pickoffs for obtaining the angle information. The disadvantage of that method mainly lies in the asymmetric structure of the rotor which has the result that any displacement of the center of rotation of the rotor results in a relatively important measuring error. Added to this, the method requires a core that covers the entire angular range. Especially when employed as a hollow-shaft sensor, a very large and expensive core is required. Contrary to optical systems, determining the effective center of such a system is quite difficult with such a system because of the nature and propagation of the magnetic field lines in such systems. In addition to the requirement of having the circuit board exactly positioned, a precise bearing system is also necessary.
In many applications it is possible either not at all, or only at high expense, for example by the use of additional bearings, to align the rotor exactly with the stator. In practice, eccentricities and axial displacements of more than xc2x11 mm may be encountered. In the case of asymmetric systems it is then necessary to work with large diameters, which frequently leads to considerable difficulties in use.
DE 197 57 689 A1 describes an inductive measuring transducer which uses a conductor loop and a resistor network to scan a movable alternating magnetic field and derive a position-dependent output voltage. While that system is well suited for measuring limited displacement and angular ranges, it can be adapted to a continuous measuring range of 360xc2x0 only with considerable additional input.
Contrary to the known solution, the invention proposes to determine the measuring voltage by the steps of inducing a voltage, by one or more inductive sensor components mounted on a rotor, which cover only a small portion of a rotating conductor path, in a closed circular conductor loop extending along the path described by an inductive pickup element, and forming a function, defined by the position and value of different resistors, of the voltages encountered at the connection points of the different resistors and the conductor loop, by resistors distributed and connected along the periphery of the conductor path, the other ends of such resistors being connected one with the other in different groups. For example, a mean value of the voltage over an angular range can be obtained at a connection point if resistors with equal resistance values, arranged at equal spacings over that angular range, have their one ends connected to the conductor loop and their other ends connected to the connection point of the resistor group from which the voltage can be picked off. In order to permit the angle to be clearly determined over 360xc2x0, it is necessary to obtain at least two different voltage curve shapes, related to the angular position of the rotor. This is achieved by forming a plurality of resistor groups connected to different portions of the conductor loop.
When a single inductive sensor component is used, an alternating voltage is induced in the conductor loop over the angle filled by the air gap of the relatively small sensor component. That voltage produces in the conductor loop a current which, due to the inductive and ohmic resistance of the conductor loop, leads to a uniform voltage drop over the circumference. It can be shown here that by using resistor groups, each defining the mean value of a quadrant of the conductor path, a voltage curve shape as necessary for determining a clear angular value over a range of 360xc2x0 is obtained.
If a single sensor component is used only, any displacement of the center of rotation of the rotor relative to the center of the stator in the direction of movement of the inductive sensor may, however, lead to measuring errors. Similarly, a not perfectly uniform behavior of the impedance of the conductor path over the circumference may cause additional measuring errors.
According to a further embodiment of the invention, this is avoided by using two inductive sensor components, offset by 180xc2x0, to induce voltages of opposed phase and equal amplitude in the conductor loop. The conductor loop then constitutes an electric circuit with two opposed voltage sources, where no differential voltage is present and, thus, no current flows between the voltage sources. Between the different sensor components, a conductor loop carries a voltage of constant amplitude, while the oppositely arranged portion of the conductor loop carries a voltage of opposed phase and equal amplitude. Due to the fact that there is no current flow in the conductor loop, no current drop produced by the impedance of the conductor loop will be encountered, either. Thus, the impedance of the conductor loop does not enter into the measuring result.
Any displacement of the center of rotation of the rotor relative to the stator in or against the direction of movement of the two oppositely arranged inductive sensors will result in opposed errors which largely balance each other out. It is, thus, possible to build up angle sensors especially for applications that do not have a bearing system of their own and/or where assembly tolerances are high.
A corresponding magnetic potential and, thus, a corresponding excitation current is required for generating the field in the air gap of the sensor component.
In principle, excitation by a winding connected directly to an oscillator would seem possible. In practice, this is however not practicable in most of the cases because of the movable lines required in this case and the limited angle of rotation.
In the case of a single core, excitation can be achieved by simple means. A core with high magnetic permeability, provided with an air gap, is used as an inductive sensor. The core encloses the turns of a concentric coil and the conductor loop for deriving the measured values. The core is designed in such a way that the main part of the magnetic flux produced by the excitation coil is caused to flow through the air gap of the core, with the result that an alternating current is induced in the conductor loop in the area of the core, which then induces in the conductor loop a voltage distribution depending on the position of the sensor relative to the pickup.
The stray flux of the excitation coil causes the measuring result to be weakened and leads to an additional measuring error. This effect can be greatly reduced by short-circuiting rings of low impedance which, while being efficiently inductively coupled to the excitation coil, omit the area of the core of the inductive sensor. A ring of that kind is to be regarded as short-circuited secondary winding of the primary coil. The current flowing in that ring is opposed to the primary current of the excitation coil and largely cancels out the field outside the core.
A current flowing in such a secondary winding may also serve to feed the one or more inductive sensors. It is, thus, no longer necessary for the core of the sensor to enclose the turns of the excitation coil. This permits an especially space-saving structure to be implemented because the secondary winding can be arranged at the least possible distance from the excitation coil. Such an arrangement makes it possible, in principle, to do without any soft magnetic cores although a reduced useful signal must be accepted in that latter case.
The useful signal can be increased in this case by improving the coupling between the excitation coil and the secondary winding by a soft magnetic core that encloses the excitation coil and the secondary winding so that the magnetic field lines of the excitation winding will flow predominantly through that core and the air gap formed by the latter.
In the case of oppositely arranged sensors, direct injection of the sensor voltage from the excitation coil into the measuring cores is possible either not at all or only with extreme difficulty because an opposed voltage is required. Each of the cores must be fed through a winding of its own. It is necessary for this purpose to provide an intermediate circuit on the rotor, as has been described before. This can be achieved by a secondary coil, coupled inductively to the concentric excitation coil, which is supplied with an alternating voltage and which has its terminals connected to the windings of the measuring cores in opposite winding directions, it being also possible to configure the coils as simple conductor loops.
The operating principle of an inductive measuring transducer with closed conductor loop can be used with advantage also for displacement measurements. The closed conductor loop is then configured approximately as a rectangle, to the long sides of which the resistors are connected in two or more groups. Different embodiments are possible in this respect. If a single core is used, two resistor groups are provided, which are connected on that long side of the measuring loop that extends in the measuring direction. The measured voltage then is the differential voltage between the two resistor groups. Between the sides of the measuring loop that extend crosswise to the measuring direction, a differential voltage is countered which is independent of the position of the inductive sensor and that corresponds to half the voltage induced by the inductive sensor. The latter can then be used as reference voltage for forming a ratio between position-dependent and position-independent voltage (ratiometric method), whereby the effect of factors that influence the output voltage, such as temperature, coil resistance, core geometry, can be largely eliminated.
Excitation of the inductive sensor may be effected by a coil wound directly onto the core of the sensor. This arrangement prevents any interfering stray flux that might influence the measuring result.
In many cases it is, however, not possible to wind the coil directly onto the core of the inductive sensor, for example because otherwise movable lines would be required which are very much prone to failure. Excitation of the inductive sensor can then be achieved by a coil that extends over the measuring length, with the core enclosing the turns and the conductor loop.
When the excitation coil is arranged directly below the measuring loop, then the stray flux encountered will be relatively strong. Ideally, this remains without effect, initially, although any faults in the resistance profile over the measuring loop make themselves felt more strongly. It is, therefore, advantageous to give the measuring loop a configuration where the induced voltages emanating from the excitation coil and the unavoidable stray flux balance out each other. This can be achieved by a design where the measuring loop extends in opposite directions, i.e. where the measuring loop consists of two line sections whose ends are connected by low-resistance lines in such a way that the beginning of each line section is connected to the end of the respective other line section. Another possibility consists in using a normal rectangular loop and dividing the coil into two partial coils arranged one beside the other and wound in opposite senses.
As in the case of angle-measuring systems a design using two cores, that induce opposed voltages in the conductor loop, is also possible. In this case, no current will flow in the conductor loop so that the resistance of the conductor loop will not enter into the measuring results.
An increase of the measuring range can be achieved also by an arrangement comprising two measuring cores associated to the two oppositely arranged line sections of the measuring loop. When the measuring core leaves the area of the conductor loop, the second measuring core will enter the oppositely arranged section of the measuring loop. It is, however, of advantage in this case to provide four resistor groups and to perform the evaluation in a manner similar to that of an angle sensor, with two characteristics displaced by 90xc2x0. The measuring length can be further extended by additional cores, in which case periodically repeating output signals will be obtained so that the output signal becomes ambiguous. Accordingly, additional measures, known as such, will be necessary in this case for determining the absolute position.
In the description that follows, the operating principles described above will be explained by way of examples.